The present invention relates to an electronic circuit for reducing switching losses in current-driven power devices. The invention relates, particularly, but not exclusively, to an electronic circuit comprising a power device connected to an electric load and having at least one control terminal arranged to receive a predetermined drive current value.
In most electronic applications, and especially those including high-frequency operations, there is a need for reducing or minimizing the power losses during the switching phases of the power devices. This is desired to keep the operating temperature of the power devices as low as possible.
The problems involved in reducing switching losses at turn-off are independent from the kind of load that is connected to the power device. On the contrary, in applications involving just resistive loads, and with currents that will quickly attain their maximum value, the turn-on phase must be considered.
High reactivity is demanded in the power device at turn-on, to prevent the appearance of elevated currents and/or voltages. It is in this perspective that the art has long been looking for circuit schemes that can enhance or optimize both the dynamic saturation and the performance of power devices at turn-on and turn-off.
Many of the power devices formed by discrete components and intended for driving electric loads comprise bipolar transistors. A speed-up capacitor connected in parallel with a base resistor Rb is commonly used for driving the bipolar transistors. At the turn-on phase, the capacitor supplies a current pulse that, once added to the current from the base resistor, will optimize the saturating volt age of the power transistors. At the turn-off phase, the speed-up capacitor will output a current pulse from the base terminal of the bipolar transistor. This current pulse contributes to dissipating the stored charge, thus shortening the storage time.
Integrated driver circuits for driving electric loads include final power stages that are controlled by currents that are proportional to the output currents of the power stages. A maximum current is flowed through these circuits only when needed, such as when a power element arranged to produce a current for an inductive load is driven by a drive current that is proportional to the load current.
The approaches just described have certain limitations, either where discrete components or integrated circuits are provided. For instance, when the signal originates from a resistive load, whereby the current will rise to its final value immediately, the driving method based on the proportional current cannot be applied. Also, when large currents are flowed through the circuit, the approach based on the use of discrete components will require a large speed-up capacitor and, therefore, result in a substantially higher device cost.
An object of the present invention is to provide an electronic circuit effective to reduce or optimize switching losses in current-driven power devices, such a circuit having relatively simple structural and functional features to allow enhancing the turn-on and off phases of the power device to be driven, thereby addressing the limitations of the prior art.
The circuit in accordance with the invention provides a current step additional to the drive signal and effective to lower the voltage across the power device during the switching phases of the power device. One embodiment of the invention relates to an electronic circuit for reducing or optimizing switching losses in current-driven power devices. This circuit may comprise a switching power device connected to an electric load and having at least one control terminal arranged to receive a predetermined drive current value from a current generator, and a structurally independent current generator producing a further drive current portion to supply to the control terminal. The circuit also includes a controlled switch between the independent current generator and the control terminal, and a control circuit controlling the switch during the turn-on and turn-off phases of the power device.
Another embodiment of the invention also relates to an electronic circuit for reducing switching losses in current-driven power devices. This circuit may include a power switching device connected to an electric load and having at least one control terminal receiving a first drive current value produced by a first current generator, and a first controlled switch between the first current generator and the control terminal. The circuit may also include a second current generator producing a second drive current value to be supplied to the control terminal, a second controlled switch between the second current generator and the control terminal, and a control circuit controlling the first and second switches during the turn-on and turn-off phases of the power device.